This invention relates to the formation of toner images on an image member. Although not limited thereto, the invention is particularly useful in a method and apparatus for forming two or more different color toner images on a single frame of an image member.
U.S. Pat. No. 5,001,028 to Mosehauer et al is representative of a number of references describing a process in which a photoconductive image member is uniformly charged and imagewise exposed to create an electrostatic image. Dry toner is applied to the electrostatic image to create a toner image. Usually in this process, discharged area development is used. Thus, the toner applied is of the same polarity as the electrostatic image. Deposits in the areas of lowest charge (the discharged areas) form a toner image having a density which is greatest in the portions of the image receiving the greatest exposure. Without fixing the first toner image, the image member is usually uniformly charged, again with a charge of the same polarity as the original image and imagewise exposed to form a second electrostatic image generally in the portions of the image member not covered by the first toner image. The second electrostatic image is toned, again with a toner of the same polarity as the electrostatic image but of a color different from the first toner image, to create a second toner image. The process can be repeated with a third electrostatic image toned by a third color toner to create a three color image, etc. The two (or more) color images all have the same polarity and are easily transferred in a single step to a receiving sheet and fused, also in a single step.
Although the process is not necessarily limited to such applications, it is most commonly used to provide accent color prints or copies with laser or LED printhead electronic exposure. All commercial applications known to me use electronic exposure and discharged area development.
The process has a number of advantages in multiple color applications. It eliminates the troublesome, inaccurate and/or expensive steps used in registering images at a transfer station. If it uses separate exposure stations for each image, it can produce multiple color output at the same speed as single color output.
It is important that the second and subsequent toning steps not disturb the previous toner images. Otherwise, toner from the first toner image, unintentionally removed ("scavenged") from the PC, gets mixed into the second development station and toner from the second development station is deposited on the first toner image ("overtoning"). The relative seriousness of scavenging and overtoning is dependent upon the order of colors. In a system in which a lighter color is deposited first, and a darker color later, overtoning is more serious than scavenging. However, in a system in which a darker color, for example, black, is deposited first and the lighter color is deposited second, scavenging of the dark color into the light color toning station is a much more serious problem. (Overtoning could occur as a result of scavenging with the second color replacing the scavenged first color toner.)
Scavenging can be greatly reduced by using projection toning for toning the second and subsequent electrostatic images. For greatest deposition of toner using projection toning, an AC signal is applied to the toning field; see, for example, U.S. Pat. No. 4,803,518 to Haneda et al, granted Feb. 7, 1989 and, particularly, U.S. patent application Ser. No. 07/065,249, filed May 20, 1993 to Kaukeinen et al, entitled IMAGE FORMING METHOD AND APPARATUS and other references referred to therein.
Whether or not an AC signal is used, a DC component (V.sub.b') of the development field in the second development step is an important parameter in controlling the process. U.S. Pat. No. 4,860,048 to Itoh et al, issued Aug. 22, 1989, suggests using an electrometer to examine toner voltages in a binary system. The voltage readouts can be used to adjust V.sub.b' and also the second charging (sometimes called "recharging") step. The electrometer is positioned after the second exposure step and appears to examine the image, apparently isolating the voltage on the toner from the rest of the first toner image. This is then used to adjust V.sub.b' to prevent toning of either the background or the first toner image.
Other references suggest careful control of the recharging step, also in binary systems, to prevent scavenging and overtoning; see, for example, U.S. Pat. No. 4,611,901 to Kohyama et al, issued Sep. 16, 1986; U.S. Pat. No. 4,819,028 to Abe, issued Apr. 4, 1989; and U.S. Pat. No. 4,927,724 to Yamamoto et al, issued May 22, 1990.
U.S. Pat. No. 5,182,599 to Kinoshita, issued Jan. 26, 1993, suggests forming a conventional process control patch whose density is read for control of toner concentration, developer bias and charging in a multicolor, single frame system.
U.S. Pat. No. 5,208,632 to Hurwitch et al, issued May 4, 1993, uses a pair of electrometers to read six electrostatic patches for control decisions in a single exposure accent color system.